Lithium silicate cathodes for lithium-ion batteries

ABSTRACT

An improved nanocomposite cathode material for lithium-ion batteries and method of making the same. The nanocomposite cathode material includes lithium iron silicate based nanoparticles with a conductive matrix of graphene sheets. The nanoparticles may be doped with at least one anion or cation.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/814,396, filed Mar. 6, 2019, the disclosure of which is hereby expressly incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to lithium-ion (Li-ion) batteries and, more particularly, to lithium iron silicate (Li₂FeSiO₄) based cathodes for Li-ion batteries.

BACKGROUND OF THE DISCLOSURE

Since the introduction of Li-ion batteries, the demand for increasingly higher specific capacity and specific energy batteries has steadily increased with the advance of portable electronics, electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. Likewise, the need for alternative fuel sources has grown over the last decades, due to such factors as the rise of oil prices, the increase in global population, and the pollution generated by internal combustion vehicles. As world population continues to grow, so will the number of vehicles and the demand for more efficient vehicles that require fewer natural resources and generate less pollution.

Advancement in battery technology has made the dream of replacing internal combustion engines with electric motors a reality, reducing the consumption of liquid hydrocarbon fuels. Implementation of battery-powered EVs still faces stiff opposition as they carry a higher cost, still have limited range, and suffer weight parity issues when compared to traditional internal combustion vehicles. Further, the batteries of choice, Li-ion batteries, suffer from short cycle lives and exhibit significant degradation over time, making battery powered vehicles less attractive.

The cathode materials of most Li-ion batteries include transition metal compounds, oxides, or complex oxides. Such transition metal compounds have layered crystal structures (e.g., lithium cobalt oxide (LiCoO₂)), spinel crystal structures (e.g., lithium manganese oxide (LiMn₂O₄)), or olivine crystal structures (e.g., lithium iron phosphate (LiFePO₄)). The transition metal cations typically display four- and/or six-fold coordination with oxygen anions, anionic clusters, or ligands, classified into the full octahedron, full tetrahedron as well as octahedron/tetrahedron hybrid structures.

In operation, Li⁺ ions are inserted via an electrochemical intercalation reaction. While Li⁺ ions occupy the space between adjacent layers or unoccupied octahedral or tetrahedral sites, an equal number of electrons enter the available d orbitals of the transition metal cations in the host crystal. Essentially, the oxidation state of metal ions keeps change with the deinsertion accompanying the phase change of these compounds while the Li⁺ ions remain in ionic state. These materials have some common characteristics: (1) chemical stability, (2) structural stability, and (3) channels allowing the effective diffusion of Li⁺ ions within the solid oxides. The chemical stability of the cathode material ensures that the host of the cathode does not decompose during the delithiation process while structural stability allows the repeated deintercalation of Li⁺ ions into the lattices of the host materials. Channels within the materials lead to the high-rate delithiation process within the materials, which in turn is essential for the high rate performance of Li-ion batteries.

The performance characteristics of various cathode materials are summarized in Table 1 below.

TABLE 1 LiNi_(x)Mn_(y)Co_(z)O₂ Cathode Type LiMn₂O₄ LiCoO₂ LiFePO₄ (NMC) Li₂FeSiO₄ Potential Theoretical 4.2 4.0 3.5 3.7 3.5 (V vs Li/Li⁺) Practical 4.0 3.8 3.3 3.5 3.0 No. of Li⁺ Theoretical 1 1 1 1 2 Intercalated Practical 1 0.5 1 0.5 2 Specific Theoretical 148 274 175 280 331 Capacity Practical 120 145 165 170 305 (mAh/g) Specific Energy Theoretical 622 1096 613 1036 1158 (Wh/kg) Practical 480 551 545 595 915 Relative Cost 30 60 30 65 30 ($/kg)

A particularly attractive cathode material for Li-ion batteries is Li₂FeSiO₄, which has a high specific capacity with two Li⁺ ion insertion and a high specific energy (See Table 1). Additionally, Li₂FeSiO₄ is chemically stable, safe, nontoxic, and affordable (See Table 1). However, the practical applications of V₂O₅ have been limited due to: (1) low electrical conductivity due to its tetrahedron structure, (2) slow Li⁺ ion diffusion, (3) structural instability during the repeated delithiation processes due to weaknesses in its tetrahedron structure, and (4) interfacial instability due to the high charging voltage for driving the Li⁺ ions into the stable FeSiO₄ network. Efforts have been made to improve the performance of Li₂FeSiO₄, including porous nanostructures, cation doping in crystals, carbon coating or fabrication of carbonaceous matrices, incorporation of reduced graphene, conductive inorganic compounds, or organic polymers. Although these efforts have improved the performance of Li₂FeSiO₄ to a certain degree, these efforts have not significantly improved the specific capacity, structural stability, and cycle life to the level of practical application.

For these reasons, there is a need for a cathode material for Li-ion batteries that takes advantage of the benefits of Li₂FeSiO₄ while addressing its inherent drawbacks in a comprehensive manner.

SUMMARY

The present disclosure provides an improved nanocomposite cathode material for lithium-ion batteries and method of making the same. The nanocomposite cathode material includes Li₂FeSiO₄-based nanoparticles with a conductive matrix of graphene sheets. The nanoparticles may be doped with at least one anion or cation.

According to an embodiment of the present disclosure, an electrode material is provided including a conductive matrix comprising a plurality of graphene sheets, and a plurality of Li₂FeSiO₄-based nanoparticles coupled to the conductive matrix.

According to another embodiment of the present disclosure, a method is provided for making an electrode material, the method including: preparing a solution containing a lithium oxide, a silicon oxide, and iron; adding graphene oxide to the solution; heating the solution to produce a plurality of Li₂FeSiO₄-based nanoparticles over the graphene oxide; and sintering the nanoparticles to reduce the graphene oxide to graphene and produce a nanocomposite material.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this disclosure, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic view of a Li-ion battery of the present disclosure;

FIG. 2 is a schematic view of an exemplary Li₂FeSiO₄-based/Graphene nanocomposite material of the present disclosure;

FIG. 3 is a flow chart illustrating an exemplary method for preparing the Li₂FeSiO₄-based/Graphene material;

FIGS. 4A-4C are transmission electron microscopy images of Li₂FeSiO₄-based particles alone, with graphene, and with graphene and fluorine dopants, respectively, in accordance with the examples;

FIGS. 5A-5D graphically illustrate electrochemical test results for electrodes made from the Li₂FeSiO₄-based particles of FIGS. 4A-4C, in accordance with the examples;

FIG. 6 graphically illustrates capacity data at different fluorine doping ratios in accordance with the examples;

FIG. 7 graphically illustrates cycle life data at different fluorine doping ratios in accordance with the examples;

FIGS. 8A-8C graphically illustrate electrochemical test results for electrodes made with graphene and fluorine dopants in accordance with the examples;

FIGS. 9A-9C graphically illustrate electrochemical test results for electrodes made with graphene and manganese dopants in accordance with the examples;

FIG. 10 graphically illustrates capacity data at different manganese doping ratios in accordance with the examples;

FIG. 11 graphically illustrates cycle life data at different niobium doping ratios in accordance with the examples; and

FIG. 12 graphically illustrates cycle life data at different titanium doping ratios in accordance with the examples.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION

FIG. 1 provides a Li-ion battery 100 including an anode 102 coupled to an anode current collector 103, a cathode 104 coupled to a cathode current collector 105, and an electrolyte-filled separator 106. In one particular example, the anode 102 comprises Li metal or graphite and the electrolyte comprises a solution of LiPF₆ in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), but sulfone-based, ionic liquid-based, nitrile-based, or other electrolytes can also be used. The illustrative battery 100 is a pouch cell, but the battery 100 may also be a cylindrical cell, a coin cell, or a prismatic cell, for example. The battery 100 may be configured for use in a portable electronic device, an electric vehicle, an energy storage device, or other electronic devices.

FIG. 2 provides an improved Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110 for cathode 104 of battery 100 (FIG. 1). Advantageously, the Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110 may have an initial specific energy of 600 Wh/kg, may have a cycle life of at least 1,000 cycles, and may lack cobalt, nickel, and manganese.

As shown in FIG. 2, the Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110 includes Li₂FeSiO₄-based nanoparticles 112 (e.g., nanorods) formed upon a conductive matrix of graphene sheets 114. The graphene sheets 114 may improve the electrical conductivity of the nanocomposite cathode material 110 compared to the Li₂FeSiO₄-based nanoparticles 112 alone. Additionally, the graphene sheets 114 may provide a structural matrix to anchor and stabilize the Li₂FeSiO₄-based nanoparticles 112 and reduce grain stress during charge/discharge cycling, leading to better cycle life. As shown in FIG. 2, each graphene sheet 114 includes a single-layer of graphene with sp²-bonded carbon atoms arranged in a honeycomb crystal structure and can be viewed as an individual atomic plane of a graphite structure. Each carbon atom in the graphene uses 3 of 4 valence band (2s, 2p) electrons (which occupy the sp² orbits) to form 3 covalent bonds with the neighboring carbon atoms in the same plane. Each carbon atom in the graphene contributes its fourth lone electron (occupying the p_(z) orbit) to form a delocalized electron system, a long-range π-conjugation system shared by all carbon atoms in the graphene plane. Such a long-range π-conjugation in graphene yields extraordinary electrical, mechanical, and thermal properties in the nanocomposite cathode material 110. The nanocomposite cathode material 110 exhibits improved intraparticle electronic conduction because of good electrical conductivity of graphene, and Li⁺ ion diffusion is improved because diffusion length is shortened. Furthermore, the small grain size of the Li₂FeSiO₄-based nanoparticles 112 in the nanocomposite cathode material 110 reduces internal stress, leading to better structure stability and cycle life. According to an exemplary embodiment of the present disclosure, the nanocomposite cathode material 110 may comprise about 1 wt. % to about 10 wt. % graphene, more specifically about 1 wt. % to about 5 wt. % graphene, more specifically about 2 wt. % graphene. The graphene content should be sufficiently low to maintain the graphene as single sheets and avoid re-stacking. Additional information regarding the incorporation of graphene sheets 114 is disclosed in U.S. Publication No. 2015/0380732, the disclosure of which is expressly incorporated herein by reference in its entirety.

In certain embodiments, the graphene sheets 114 may be modified with one or more functional groups (e.g., —OH, —COOH, —NH). For example, the functional groups may be covalently grafted onto the surface of the graphene sheets 114 through diazonium salt via a diazonium reaction. The diazonium reaction-based functionalization may provide a simple and cost-effective way to transform the pure graphene sheets 114 into hierarchical and functional materials that can provide the desired properties (i.e. hydrophobicity, Li⁺/e⁻ conductivity, Li⁺ diffusivity, nanoparticle dispersion, local electric field, etc.) to enhance binding with the adjacent Li₂FeSiO₄-based nanoparticles 112.

The Li₂FeSiO₄-based nanoparticles 112 may also include one or more optional dopants, including anion dopants (X) and/or cation dopants (Y). The dopants X, Y, may alter the crystalline structure and, consequently, the electrochemical performance of the Li₂FeSiO₄-based nanoparticles 112. The types of dopants X, Y, the doping order, and the amount of dopants X, Y may be varied to achieve a desired specific capacity/energy and cycle life. According to an exemplary embodiment of the present disclosure, the Li₂FeSiO₄-based nanoparticles 112 may have an anion doping ratio of anion dopants X to oxygen (O) and/or a cation doping ratio of cation dopants Y to iron (Fe). The doping ratios may be about 30% or less by weight, more specifically about 2% by weight to about 20% by weight, and more specifically about 2% by weight to about 10% by weight.

Examples of suitable anion dopants X include halogen ions, such as fluorine ions (F⁻), chlorine ions (Cl⁻), and bromine ions (Br⁻). Such anion dopants X may have a larger electronegativity than oxygen (O) and may reduce the formation of ligand holes during delithiation, which may stabilize the crystalline structure by reducing the extent of an O²⁻→O₂ ²⁻ reaction during delithiation. Also, such anion dopants X may also have a high redox potential (e.g., 2.87 V for F), which may help to increase the discharge potential of the nanocomposite cathode material 110 in the Li-ion battery 100 (FIG. 1).

Examples of suitable cation dopants Y include titanium ions (Ti⁺⁴), manganese ions (Mn⁺²), copper ions (Cu⁺²), terbium ions (Tb⁺³), niobium ions (Nb⁺⁵), and molybdenum ions (Mo⁺⁴). Such cation dopants Y may improve the structure stability of the Li₂FeSiO₄-based nanoparticles 112 by enhancing the coupling effect among the Li₂FeSiO₄-based tetrahedra via strong d-orbital hybridization. In this way, the dopants Y may function like springs to contain the tetrahedra and prohibit structural fracture. Consequently, the dopants Y may achieve much improved cycle life.

The Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110 may have the following Formula (F-I):

Li₂Fe_(1−d)Y_(c)SiO_(4−b)X_(a)/Graphene  (F-I)

wherein:

-   -   X=anion dopant;     -   Y=cation dopant;     -   a≥0;     -   b=f(a)     -   c≥0; and     -   d=f(c).

Examples of suitable Li₂FeSiO₄-based/Graphene nanocomposite cathode materials 110 are listed in Table 2 below.

TABLE 2 Material X b Y d Li₂FeSiO₄/Graphene — 0 — 0 Li₂FeSiO_(4−a/2)Fa/Graphene F a/2 — 0 Li₂Fe_(1−c)Mn_(c)SiO₄/Graphene — 0 Mn c Li₂Fe_(1−c)Cu_(c)SiO₄/Graphene — 0 Cu c Li₂Fe_(1−2.5c)Nb_(c)SiO₄/Graphene — 0 Nb 2.5c Li₂Fe_(1−2.5c)Nb_(c)SiO_(4−a/2)F_(a)/Graphene F a/2 Nb 2.5c

FIG. 3 provides an exemplary method 200 for synthesizing the Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110 (FIG. 2) and constructing the cathode 104 (FIG. 1). The method 200 may involve a hydrothermal synthesis process.

In step 202, a first solution is prepared including a lithium oxide, specifically lithium hydroxide (LiOH), and a silicon oxide, specifically silica (SiO₂), in a suitable solvent such as distilled water. The SiO₂ may be provided in the form of nanoparticles, also referred to herein as nano-SiO₂.

In step 204, a second solution is prepared including an iron compound, specifically iron dichloride (FeCl₂), in a suitable solvent such as distilled water.

In step 206, the first and second solutions are combined. In certain embodiments, the second solution is added dropwise to the first solution. The combined solutions may be stirred together for about 30 minutes, about 1 hour, or longer before proceeding to the next step.

In step 208, a graphene oxide (GO) solution is added to the combined solution from step 206. The GO solution may be prepared using a modified Hummer's method, as disclosed in Example 1 below, for example. In certain embodiments, the GO solution is added dropwise to the combined solution from step 206. The resulting solution may be stirred together for about 30 minutes, about 1 hour, or longer before proceeding to the next step.

In step 210, a salt of any desired dopant X, Y is added to the resulting solution from step 208. Examples of suitable salts include ammonium fluoride (NH₄F) for the F-dopant, cupric chloride (CuCl₂) for the Cu-dopant, and niobium hydroxide (Nb(OH)₅) for the Nb-dopant. The dopant X, Y may be present in a desired concentration relative to the Li₂FeSiO₄.

In step 212, the solution is reacted to produce optionally doped, Li₂FeSiO₄-based nanoparticles 112 (FIG. 2) over the GO. This reacting step 210 may be a hydrothermal step that is performed in an autoclave or another suitable heated environment. The temperature of the reacting step 210 may be about 160° C., about 180° C., about 200° C., or more, but this temperature may vary. The duration of the reacting step 210 may be about 5 hours, about 10 hours, about 15 hours, or more, but this duration may vary. After the reacting step 210 is completed, the autoclave may be allowed to return to room temperature before removing the products.

The reacting step 212 may produce Li₂FeSiO₄-based nanoparticles 112 upon the GO according to Reactions (R-I) thru (R-III), represented overall by Reaction (R). The Reaction (R) may be modified as appropriate to incorporate any desired dopants.

2LiOH+SiO₂→Li₂SiO₃.H₂O  (R-I)

FeCl₂+2LiOH→Fe(OH)₂+2LiCl  (R-II)

Fe(OH)₂+Li₂SiO₃.H₂O→Li₂FeSiO₄+2H₂O  (R-III)

4LiOH+SiO₂+FeCl₂→Li₂FeSiO₄+2H₂O+2LiCl  (R)

In step 214, the Li₂FeSiO₄-based nanoparticles 112 produced during the reacting step 212 are separated and cleaned. This step 214 may involve: removing the precipitated nanoparticles 112 from the aqueous solution, such as by filtering or drying; rinsing the nanoparticles 112 with deionized water or another suitable rinsing agent; and drying the nanoparticles 112, such as by subjecting the nanoparticles 112 to an elevated temperature and/or a vacuum environment for several hours or more.

In step 216, the Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110 is formed by sintering the Li₂FeSiO₄-based nanoparticles 112 upon the GO, which reduces the GO to graphene 114 (FIG. 2). The sintering step 112 may be performed in an inert atmosphere, such as argon (Ar). The temperature of the sintering step 216 may be about 500° C., about 600° C., about 700° C., or more, but this temperature may vary. The duration of the sintering step 216 may be about 5 hours, about 10 hours, about 15 hours, or more, but this duration may vary.

In step 218, the cathode 104 (FIG. 1) is constructed using the Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110 from the sintering step 216. The constructing step 218 may involve preparing a slurry. In one example, the slurry contains about 80 wt. % Li₂FeSiO₄-based/Graphene nanocomposite cathode material 110, about 10 wt. % polyvinylidence difluoride (PVDF), and about 10 wt. % carbon black. Next, the slurry may be sprayed onto or otherwise applied to the cathode current collector 105, such as a 10 μm thick aluminum (Al) foil. Then, the cathode 104 may be dried in a vacuum oven, such as at a temperature of about 90° C. and a duration of about 24 hours.

While this invention has been described as having exemplary designs, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

Examples 1. Preparation of GO Solution

A GO solution was prepared using a modified Hummer's method. 2 grams of graphite flakes were mixed with 10 mL of concentrated H₂SO₄, 2 grams of (NH₄)₂S₂O₈, and 2 grams of P₂O₅. The obtained mixture was heated at 80° C. for 4 hours under constant stirring. Then the mixture was filtered and washed thoroughly with DI water. After drying in an oven at 80° C. overnight, this pre-oxidized graphite was then subjected to oxidation using the Hummer's method. 2 grams of pre-oxidized graphite, 1 gram of sodium nitrate and 46 mL of sulfuric acid were mixed and stirred for 15 minutes in an iced bath. Then, 6 grams of potassium permanganate was slowly added to the obtained suspension solution for another 15 minutes. After that, 92 mL DI water was slowly added to the suspension, while the temperature was kept constant at about 98° C. for 15 minutes. After the suspension has been diluted by 280 mL DI water, 10 mL of 30% H₂O₂ was added to reduce the unreacted permanganate. Finally, the resulted suspension was centrifuged several times to remove the unreacted acids and salts. The purified GO were dispersed in DI water to form a 0.2 mg/mL solution by sonication for 1 hour. Then the GO dispersion was subjected to another centrifugation in order remove the un-exfoliated GO. The resulted GO dilute solution could remain in a very stable suspension without any precipitation for a few months.

2. Synthesis of Li₂FeSiO₄

First, 16 mmol LiOH and 4 mmol nano-SiO₂ were dissolved in 30 mL distilled water to produce Solution 2-A, and 4 mmol FeCl₂.4H₂O was dissolved in another 20 mL distilled water to produce Solution 2-B. After 1 h stirring, the aqueous Solution 2-B was added dropwise in Solution 2-A with continued stirring for 4 h to produce Solution 2-C.

Then, Solution 2-C was transferred into a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li₂FeSiO₄.

Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The pure Li₂FeSiO₄ was sintered at 600° C. for 10 h in argon (Ar) atmosphere.

Using transmission electron microscopy (TEM), the Li₂FeSiO₄ particles measured about 15 nm in diameter (FIG. 4A).

3. Synthesis of Undoped Li₂FeSiO₄/Graphene

First, 16 mmol LiOH and 4 mmol nano-SiO₂ were dissolved in 30 mL distilled water to produce Solution 3-A, and 4 mmol FeCl₂.4H₂O was dissolved in another 20 mL distilled water to produce Solution 3-B. After 1 h stirring, the aqueous Solution 3-B was added dropwise in Solution 3-A with continued stirring for 4 h to produce Solution 3-C.

Next, a 5 mg/mL GO gel was prepared according to Example 1. 20 mL of the GO gel was dropped in Solution 3-C with continued stirring for 1 h.

Then, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li₂FeSiO₄ upon the GO.

Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The Li₂FeSiO₄/GO was sintered at 600° C. for 10 h in Ar atmosphere to reduce the GO to graphene.

Using TEM, the undoped Li₂FeSiO₄/Graphene particles appeared as nanorods and measured about 5 nm in diameter and about 10-30 nm in length (FIG. 4B).

4. Synthesis of 2% Mn-Doped Li₂Fe_(0.98)Mn_(0.02)SiO₄/Graphene

First, 16 mmol LiOH and 4 mmol nano-SiO₂ were dissolved in 30 mL distilled water to produce Solution 4-A, and 3.92 mmol FeCl₂.4H₂O and 0.08 mmol MnCl₂.4H₂O as the Mn-dopant were dissolved in another 20 mL distilled water to product Solution 4-B. After 1 h stirring, the aqueous Solution 4-B was added dropwise in Solution 4-A with continued stirring for 4 h to produce Solution 4-C.

Next, a 5 mg/mL GO gel was prepared according to Example 1. 20 mL of the GO gel was dropped in Solution 4-C with continued stirring for 1 h.

Then, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li₂Fe_(0.98)Mn_(0.02)SiO₄ upon the GO according to the following Mn-doped variation of Reaction (R):

4LiOH+SiO₂+(1−d)FeCl₂ +cMnCl₂.4H₂O→Li₂Fe_(1−d)Mn_(c)SiO₄+(4c+2)H₂O+2(1−d+c)LiCl

wherein:

-   -   c=0.02; and     -   d=c=0.02.

Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The Li₂Fe_(0.98)Mn_(0.02)SiO₄/GO was sintered at 600° C. for 10 h in Ar atmosphere to reduce the GO to graphene. The doping ratio of Mn/Fe was 2% by weight, calculated as (0.02*54.938)/(0.98*55.845).

5. Synthesis of 15% F-Doped Li₂FeSiO_(3.76)F_(0.48)/Graphene and 6% F-Doped Li₂FeSiO_(3.9)F_(0.2)/Graphene

First, 16 mmol LiOH and 3.76 mmol nano-SiO₂ were dissolved in 30 mL distilled water to produce Solution 5-A, and 4 mmol FeCl₂.4H₂O was dissolved in another 20 mL distilled water to produce Solution 5-B. After 1 h stirring, the aqueous Solution 5-B was added dropwise in Solution 5-A with continued stirring for 4 h to produce Solution 5-C.

Next, a 5 mg/mL GO gel was prepared according to Example 1. 20 mL of the GO gel was dropped in Solution 5-C with continued stirring for 1 h. In addition to the GO, 0.48 mmol NH₄F as the F-dopant was added to Solution 5-C and stirred for 5 mins.

Then, the mixture was quickly transferred to a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li₂FeSiO_(3.76)F_(0.48) upon the GO according to the following F-doped variation of Reaction (R):

4LiOH+SiO₂+FeCl₂ +aNH₄F→Li₂FeSiO_(4−b)F_(a) +aNH₃↑+(2+b)H₂O+2LiCl

wherein:

-   -   a=0.48; and     -   b=a/2=0.24.

Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The Li₂FeSiO_(3.76)F_(0.48)/GO was sintered at 600° C. for 10 h in Ar atmosphere to reduce the GO to graphene. The doping ratio of F/O was 15% by weight, calculated as (0.48*18.998)/(3.76*15.999).

Using TEM, the F-doped Li₂FeSiO_(3.76)F_(0.48)/Graphene particles appeared as nanorods (FIG. 4C) and measured even smaller in particle size than the undoped Li₂FeSiO₄/Graphene particles (FIG. 4B).

A similar process was performed to produce F-doped particles with other doping ratios from 2% by weight to 10% by weight. For example, F-doped Li₂FeSiO_(3.9)F_(0.2)/Graphene particles were produced having a doping ratio of F/O of 6% by weight, calculated as (0.2*18.998)/(3.9*15.999).

6. Preparation of Li₂FeSiO₄—Based Electrodes

Cathodes were prepared using the various Li₂FeSiO₄-based nanocomposite cathode materials from Examples 2-5. Each material was slurried with 10% carbon black (Super P™ Conductive Carbon Black, TIMCAL) and 10% PVDF, sprayed onto a 10 μm thick Al foil, placed in a vacuum oven, and allowed to dry at 90° C. for 24 hours. The resulting cathodes were assembled into R2016 coin cells using Li metal anodes and dielectric separators with electrolytes including 1.0 M LiPF₆ in a 3:7 by weight solvent mixture of EC and EMC.

7. Electrochemical Performance of F-Doped Electrodes

The F-doped Li₂FeSiO₄-based electrodes were subjected to electrochemical testing. The 6% F-doped Li₂FeSiO_(3.9)F_(0.2)/Graphene electrodes from Example 5 (labeled F-LFSO-G) exhibited better overall performance than the undoped Li₂FeSiO₄/Graphene electrodes from Example 3 (labeled LFSO-G), which exhibited better overall performance than the pure Li₂FeSiO₄ electrodes from Example 2 (labeled LFSO-blank). The electrochemical test results are presented in FIGS. 5A-5D and are summarized in Table 3 below.

TABLE 3 Doping Discharge Ratio Capacity Cycle Life Specific Energy Diffusion Electrode Material (to O) (mAh/g) (at 600 Wh/kg) (Wh/kg; at ⅓ C) (cm²/s) Li₂FeSiO₄ 0% 188 <10 <250 Li₂FeSiO₄/Graphene 0% 247 42 <550 2.90 × 10⁻¹⁶ Li₂FeSiO_(3.9)F_(0.2)/Graphene 6% 305 63 720 2.21 × 10⁻¹²

A surprising phenomenon was seen in the specific capacity/energy of the Li₂FeSiO_(3.9)F_(0.2)/Graphene electrodes, where the specific energy increased during the first 25 cycles from 830 Wh/kg to 1020 Wh/kg before decreasing (FIG. 5B).

The capacity results for the other F-doped Li₂FeSiO₄-based electrodes referenced in Example 5 are presented in FIG. 6. The 6% F-doped electrode showed the highest capacity. However, the additional F-dopant had a negative impact on cycle life, as shown by comparing the 4% F-doped results to the 6% F-doped results in FIG. 7.

The 6% F-doped electrode was subjected to further electrochemical testing at 0.1C, and the results are presented in FIGS. 8A-8C.

8. Electrochemical Performance of Mn-Doped Electrodes

The 2% Mn-doped Li₂FeSiO₄-based electrodes from Example 4 were subjected to electrochemical testing at 0.1C, and the results are presented in FIGS. 9A-9C.

Similar Mn-doped Li₂FeSiO₄-based electrodes were prepared with different Mn-doping ratios from 0.5% by weight to 30% by weight, and the comparative capacity results are presented in FIG. 10. The 2% Mn-doped electrode showed the highest capacity.

9. Electrochemical Performance of Other Cation-Doped Electrodes

Other Li₂FeSiO₄-based electrodes were prepared with cations other than Mn at different doping ratios. The comparative capacity results are presented in Table 4.

TABLE 4 Doping Ratio Specific Capacity (mAh/g) (to Fe) Ti Cu Nb Mo 2% 259 258 271 169 5% 142 269 282 151 8% 248 286 307 236

10. Electrochemical Performance of Hybrid-Doped Electrodes

Other Li₂FeSiO₄-based electrodes were prepared with both anion and cation dopants, specifically 6% F-dopant and from 2% to 10% cation dopant. The initial cycle life data for 6% F-2% Ti hybrids and 6% F-2% Cu hybrids is presented in Table 5. The cycle life data for 6% F-2% Nb, 6% F-8% Nb, and 6% F-10% Nb hybrids is presented in FIG. 11. The cycle life data for 6% F-8% Ti hybrids is presented in FIG. 12. In general, the cation dopants may improve cycle life compared to F-dopants alone.

TABLE 5 Specific Capacity (mAh/g) Dopants 1 2 3 6%F—2%Ti 259 240 246 6%F—2%Cu 258 235 243 

What is claimed is:
 1. An electrode material comprising: a conductive matrix comprising a plurality of graphene sheets; a plurality of lithium iron silicate based nanoparticles coupled to the conductive matrix.
 2. The material of claim 1, wherein the nanoparticles include at least one dopant, the nanoparticles having the formula: Li₂Fe_(1−d)Y_(c)SiO_(4−b)X_(a) wherein: X=an anion dopant; Y=a cation dopant; a≥0; b=f(a) c≥0; and d=f(c).
 3. The material of claim 2, wherein X is selected from the group consisting of: fluorine, chlorine, and bromine.
 4. The material of claim 2, wherein Y is selected from the group consisting of: titanium, manganese, copper, terbium, niobium, and molybdenum.
 5. The material of claim 2, wherein a>0 and c>0.
 6. The material of claim 2, wherein a>c.
 7. The material of claim 2, wherein: an anion doping ratio of X to oxygen is about 0% to about 10% by weight; and a cation doping ratio of Y to iron is about 0% to about 10% by weight.
 8. The material of claim 2, wherein: X is fluorine; and b=a/2.
 9. The material of claim 2, wherein: Y is manganese or copper; and d=c.
 10. The material of claim 2, wherein: Y is niobium; and d=2.5c.
 11. The material of claim 1, wherein the material comprises about 1 wt. % to about 10 wt. % of the graphene sheets.
 12. The material of claim 11, wherein the material comprises about 2 wt. % of the graphene sheets.
 13. A battery comprising an electrode with the material of claim
 1. 14. The battery of claim 13, wherein the battery is configured for use in a portable electronic device, an electric vehicle, or an energy storage device.
 15. The battery of claim 13, wherein the electrode lacks cobalt, nickel, and manganese.
 16. A method of making an electrode material comprising: preparing a solution containing a lithium oxide, a silicon oxide, and iron; adding graphene oxide to the solution; heating the solution to produce a plurality of lithium iron silicate based nanoparticles over the graphene oxide; and sintering the nanoparticles to reduce the graphene oxide to graphene and produce a nanocomposite material.
 17. The method of claim 16, wherein the lithium oxide of the preparing step is lithium hydroxide, the silicon oxide is silica, and the iron compound is iron dichloride.
 18. The method of claim 16, further comprising adding at least one dopant to the solution before the heating step.
 19. The method of claim 16, wherein the temperature of the heating step is about 160° C. or more.
 20. The method of claim 16, further comprising constructing a cathode with the nanocomposite material. 